Polymorphism and solid solutions of trimethylammonium monocarboranes

Abstract

Metal closo-boranes have recently received significant attention as solid-state electrolytes due to their high thermal and electrochemical stability, and the weak interaction between the cat- and anion, facilitating fast ionic conductivity. Here we report a synthesis method for obtaining a novel mixed closo-carborane compound, [NH(CH₃)₃][(CB₈H₉)_0.26(CB₉H₁₀)_0.66(CB₁₁H₁₂)_0.08]. The crystal structures are investigated for [NH(CH₃)₃][CB₉H₁₀] and [NH(CH₃)₃][(CB₈H₉)_0.26(CB₉H₁₀)_0.66(CB₁₁H₁₂)_0.08], revealing that the latter forms a solid solution isostructural to [NH(CH₃)₃][CB₉H₁₀]. The compounds exhibit polymorphism as a function of temperature, and we report the discovery of four polymorphs of [NH(CH₃)₃][CB₉H₁₀] and four isostructural solid solution [NH(CH₃)₃][(CB₈H₉)_0.26(CB₉H₁₀)_0.66(CB₁₁H₁₂)_0.08], along with a high-temperature decomposition intermediate of the latter. The α-polymorph is an ordered structure, with increasing amounts of disorder for the β- and γ-polymorphs, while the high temperature δ- and ε-polymorphs at T > 476 K are fully disordered on both the cation and anion site. These new compounds may be used as precursors for new types of solid-state ionic conductors.